1. Field of the Invention
The present invention relates to wireless communication. More particularly, the present invention relates to a method for uplink and/or downlink synchronization in distributed antenna systems and an apparatus using the same.
2. Discussion of the Related Art
The next generation wireless communication system under active research aims a system capable of transmitting various kinds of information such as video and wireless data, being evolved from the initial system providing voice-oriented services. The fourth-generation wireless communication currently under development subsequent to the third-generation wireless communication aims to support high speed data transmission with 1 Gbps (gigabits per second) data rate in the downlink and 500 Mbps (megabits per second) in the uplink. The main objective of wireless communication system is to provide a plurality of users with reliable communication means independent of their location and mobility. However, any wireless communication channel always reveals non-ideal characteristics such as path loss, noise, fading due to multipath, inter-symbol interference (ISI), or Doppler effect due to mobility of a terminal. Various technologies are under development to overcome non-ideal characteristics of wireless communication channels and improve reliability thereof.
Meanwhile, data capacity for cellular wireless systems is ever increasing according to the introduction of M2M (machine-to-machine) communication; and the advent and deployment of various devices such as smart phones and tablet PCs. Various technologies are under development to meet the needs for high data capacity. For example, carrier aggregation (CA) technology and cognitive radio (CR) technology are good examples of an effort to utilize frequency bandwidth more efficiently. Also, multi-antenna technology, multi-base station collaboration technology, etc. to increase data capacity within limited frequency bandwidth are being studied. In other words, it might be the case that wireless communication systems will eventually evolve in such a way to increase node density around the user. A wireless communication system with high node density has chances of improving its performance still more through collaboration among nodes. In other words, a wireless communication system in which nodes are collaborating with each other provides much more excellent performance than the wireless communication system where each node operates as an independent base station (BS), advanced BS (ABS), Node-B (NB), eNode-B (eNB), or access point (AP).
To improve performance of a wireless communication system, applied may be a distributed multi-node system (DMNS) which is equipped with a plurality of nodes within a cell. A distributed multi-node system may include a distributed antenna system (DAS), a radio remote head (RRH), and so on. Also, under progress is a standardization effort for applying various multi-input multi-output (MIMO) techniques and collaborative communication techniques already developed or which can be applied in the future.
A distributed antenna system (DAS) refers to a system where antennas are distributed at various locations across a cell and the antennas are managed by a single base station. Meanwhile, antennas of a base station are concentrated at the center of the cell in case of the conventional centralized antenna system (CAS).
A distributed antenna system distinguishes from a femto cell/pico cell, a relay station, and an ad-hoc system. In the distributed antenna system, coverage area of each antenna is controlled by a central base station, not by the corresponding antenna itself. In this sense, the distributed antenna system is different from a femto cell or a pico cell where individual antennas form a separate network. The distributed antenna system is also different from a relay station or an ad-hoc system in that antennas can be connected through wire to each other. In addition, the distributed antenna system is distinguished from a simple repeater in that each antenna can send signals different from each other according to a command from a base station.
The purpose of the initial distributed antenna system was to transmit a signal repeatedly by installing more antennas inside a cell to achieve coverage in a radio-shaded area. In other words, securing coverage was the main purpose of the initial distributed antenna system. From a macroscopic point of view, however, a distributed antenna system may be regarded a kind of MIMO (multiple input multiple output) system in that antennas may transmit or receive a plurality of data streams simultaneously, thus being able to support one or several users. It is understood that the MIMO system satisfies technical requirements for the next-generation wireless communication due to high spectral efficiency. In view of MIMO system, a distributed antenna system provides more advantages than a centralized antenna system. For example, as distance between the user and the antenna is reduced, power efficiency is increased and high channel capacity due to low correlation and interference between antennas is achieved and communication of relatively uniform quality is secured irrespective of location of the user within a cell. In other words, a distributed antenna system providing advantages described above is suitable for implementing MIMO transmission to secure high data capacity demanded by the current and the future communication specifications.
For example, it may be necessary to perform rank-2 transmission or more for a single user equipment (UE) in the same frequency domain, which is called single user MIMO (SU-MIMO) transmission. In some situations, multi-user MIMO (MU-MIMO) transmission supporting multiple user equipment at the same time may be necessary. The necessity described above may be raised not only for the downlink but also for the uplink.
The SU-MIMO and MU-MIMO communication described in detail above are dealt with in the standard developing organizations such as the IEEE (Institute of Electrical and Electronics Engineers) 802 and the 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution) as essential topics; in practice, they are dealt with in the IEEE 802.16e/m and the 3GPP LTE release. 8/9. However, current communication specifications have been designed based on centralized antenna systems; therefore, it is difficult to apply the current communication specifications directly to distributed antenna systems incorporating advanced technology such as the MIMO technology. Due to this reason, needed are communication specifications supporting distributed antenna systems; needed are an uplink and/or downlink synchronization method which can be provided to the communication specifications and user equipment operating according to the method.